Is the Swarming rune very weak?

Question

Swarming is a new property rune for thrown weapons from Treasure Vault. It can be etched on a Thrown weapon. As a 2 action activity once per hour, you can do 4¹ damage in 30-foot cone.

I expect more from a 9th level rune, that is only usable once per hour. On this level, this is the minimum damage of martial classes (2dx+0+2, Striking rune + Str bonus + Weapon Specialization), and if they are serious about throwing, they can go as high as 13 (2d8+4+2 average). Admittedly, this damages multiple targets, but takes at least 3 just to break even.
Unlike elemental runes, the usefulness of this quickly falls behind, as the DC is fixed. By the time it gets 6 damage (Greater Striking, level 12) the expected Ref save of monsters increases by 4. When it gets to 8 damage, most monsters will critically save most of the time.

Is it really that bad, or am I missing something?

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1. Twice the damage dice, assuming you bought a Striking rune on level 4, as you should

Answer 1

It’s extremely niche.

A 30 foot cone is pretty big - theoretically up to 24 targets. Obviously, if you can get, let’s say 6 of them in there, this rune looks pretty good.

Of course, that’s pretty unlikely. And at level 9, seems especially bad. But 700 gold is pocket change at higher levels, and as a utility option to sit in a bandolier, doesn’t seem like the worst thing to leave on the sheet. It can also be an especially strong way to take advantage of specific weaknesses - applied to, say, a cold iron shuriken, it can be a potent anti-fiend AoE.

So yes, it’s weak and I usually wouldn’t recommend it. But it’s cool and I can envision some utility for a clever player who gets their hands on one.